The present invention relates to synthesising heavy hydrocarbons using the Fischer-Tropsch reaction, i.e., the production of hydrocarbons by reacting a mixture essentially containing carbon monoxide and hydrogen and possibly carbon dioxide. That mixture is also known as synthesis gas.
More particularly, the present invention relates to a process for synthesising hydrocarbons by reacting a mixture comprising at least carbon monoxide and hydrogen in the presence of a catalyst carried out in a three-phase reactor and in which the liquid Peclet number (Pel) is in the range 0 (excluded) to about 10.
Synthesising hydrocarbons by the reaction known as the Fischer-Tropsch reaction is an industrial process that is well known for the production of hydrocarbons that are essentially paraffinic, such as naphtha or gas oil fractions or heavier fractions such as waxes (long chain paraffins). Such hydrocarbons can be converted into fuels (gas oil, kerosene) and/or into lubricants in a consecutive step such as isomerising hydrocracking.
The hydrocarbons can be produced catalytically by chemical conversion of synthesis gas that is rich in hydrogen and carbon monoxide, generally obtained from natural gas or coal. Synthesis gas can also contain carbon dioxide. The pressures used are generally about 5 to about 200 bars absolute, normally about 5 to about 80 bars absolute and usually about 10 to about 60 bars absolute (10 bars=1 MPa), and the reaction temperatures are normally about 130xc2x0 C. to about 400xc2x0 C., normally about 150xc2x0 C. to about 350xc2x0 C. and usually about 200xc2x0 C. to about 300xc2x0 C.
The catalysts used in the process, and the methods for producing these catalysts are well known to the skilled person. Such catalysts can be of a variety of natures, and usually contain at least one metal from group VIII of the periodic table (groups 8, 9 and 10 of the new periodic table), preferably dispersed on a support that is usually mineral. Frequently, the catalyst contains at least one metal selected in the group consisting of iron, cobalt and ruthenium and usually selected in the group consisting of iron and cobalt.
The support is generally a porous material and usually a porous inorganic refractory oxide. By way of example, the support can be selected in the group consisting of alumina, silica, titanium oxide, zirconia, rare earths or mixtures of at least two of these porous minerals. Typically, the quantity of metal present in the catalyst is about 1 to about 100 parts by weight per 100 parts by weight of support and usually about 5 to about 50 parts by weight per 100 parts by weight of support.
The catalyst can also contain promoters such as those cited in the following patents: British patent GB-A-2 291 819, European patents EP-A-0 581 619, EP-B-0 764 465, U.S. Pat. No. 5,783,607, French patent FR-A-2 782 319, cited by way of reference, the description of which should be considered to be included in the present description by dint of this citation.
Several types of reactor can be used for the Fischer-Tropsch reaction, the catalyst being used either in an entrained bed, or in a reactor of the slurry bubble column or bubble column reactor type in which a gas is brought into contact with a liquid/very finely divided solid mixture, or slurry. The term xe2x80x9cslurryxe2x80x9d will be used in the remainder of the description to designate a suspension of solid particles in a liquid. The very high heat of reaction is normally eliminated using a cooling exchanger that is generally inside the reactor.
Fischer-Tropsch synthesis facilities also comprise separation means to separate firstly liquid hydrocarbons and secondly gaseous products that are residual or formed as secondary products during the synthesis, mainly comprising inert compounds, light gaseous hydrocarbons and the unreacted fraction of the synthesis gas.
The desired products are generally separated substantially completely from the catalyst (for example until the amount of residual catalyst is of the order of 1 to a few parts per million (ppm)), to enable its use or treatment in subsequent steps.
Typically, the quantity of solid particles of catalyst in the slurry represents 10% to 65% by weight of the slurry. These particles usually have a mean diameter in the range about 10 to about 800 microns. Finer particles may be produced by attrition, i.e., fragmentation of the initial catalyst particles.
The Fischer-Tropsch synthesis is a synthesis reaction that aims to produce essentially paraffinic hydrocarbons essentially containing more than 5 carbon atoms per molecule (C5+ hydrocarbons). This reaction is exothermic. Further, the catalyst and operating conditions are usually selected so as to minimise the formation of methane, which is not a desired product. That reaction is particularly exothermic and has a higher activation energy than the principal C5+ paraffin formation reaction.
European patent application EP-A-0 450 861 describes the use of a Fischer-Tropsch catalyst based on cobalt dispersed on titanium oxide in a slurry bubble column type reactor. Further, EP-B-0 450 860 describes a method for operating that type of reactor in an optimal manner.
Those two documents indicate that the performance of the catalysts essentially depends on the concentration of gaseous reactant (synthesis gas) in the reactor, i.e., on the partial pressure of carbon monoxide and hydrogen in the reaction zone.
In hydrodynamics terms, those documents then indicate that in a perfectly mixed reactor, such as a fully back-mixed reactor or CSTR, the composition of gaseous reactants and liquid and gaseous products and the concentration of catalyst are the same at any point in the reactor. Thus, those perfectly mixed reactors lead to the highest selectivity for C5+ hydrocarbons, but to the detriment of productivity.
In contrast, in a plug flow reactor, the partial concentration of reactant decreases along the entire length of the reaction zone, and that type of reactor results in the highest productivities to the detriment of selectivity.
EP-B-0 450 860 indicates that Peclet numbers for the gas phase of more than 10, also known as xe2x80x9cgas Peclet numbers or Pegxe2x80x9d, lead to a plug flow type operation regarding the gas phase, while gas Peclet numbers (Peg) of less than 1 correspond to systems in which the gas phase is perfectly mixed or stirred. Ideal perfectly stirred systems correspond to Peclet numbers tending towards zero. This Peclet number is equal to Peg=H ug/Dax, where H is the expansion height of the catalytic bed, ug is the space velocity of the gas and Dax is the axial dispersion coefficient of the gas phase.
The method that can produce an optimal slurry bubble column that is described in EP-B-0 450 860 comprises injecting gas at a mean superficial velocity such that the formation of slug flow is avoided, the gas superficial velocity being 0.2 (H/Dax) or more. A further condition applies to the superficial velocity of the liquid and the sedimentation rate of the solid (generally the catalyst) so that the solid is suitably fluidised in the liquid phase.
Those documents do not take thermal effects into account, nor the presence of an undesirable methanation reaction that has a large negative influence on the exothermicity and selectivity of the reaction. Too much exothermicity in the catalyst generally leads to an increase in the formation of methane, a product that is favoured by high temperatures, and a drop in activity, for example by sintering of the active phase (M. E. DRY, xe2x80x9cCatalysis Science and Technologyxe2x80x9d, Volume 1, Anderson and Boudart, pages 175 and 198).
Thus, those phenomena result in a substantial reduction in the production of C5+ hydrocarbons, usually irreversibly.
The invention concerns a process for converting hydrocarbons by reacting a mixture comprising at least carbon monoxide and hydrogen in the presence of a catalyst, usually based on a group VIII metal, carried out in a three-phase reactor and in which the liquid Peclet number (Pel) is in the range 0 (excluded) to about 10, preferably in the range from about 0.005 to about 8, more preferably in the range from about 0.01 to about 5 and highly preferably in the range from about 0.02 to about 3 or in the range from about 0.03 to about 1.
This process can control the reaction on a thermal level and encourage formation of hydrocarbons containing at least 2 carbon atoms per molecule, and can reduce the undesirable formation of methane.
The invention concerns a process for synthesising hydrocarbons preferably containing at least 2 carbon atoms in their molecule and more preferably at least 5 carbon atoms in their molecule by bringing a gas essentially containing carbon monoxide and hydrogen into contact in a reaction zone containing a suspension of solid particles in a liquid, which comprises solid particles of a catalyst for the reaction. Said suspension is also termed a slurry. The process of the invention is thus carried out in a three-phase reactor. Preferably, the process of the invention is carried out in a slurry bubble column type three-phase reactor.
The Applicant has discovered that it is important to be able to control the hydrodynamics of the liquid if thermal transfers are to be controlled in the reaction zone, as well as the reaction itself.
In the process of the invention, it is the reactant dissolved in the liquid phase that comes into contact with the catalyst in suspension in said phase and which reacts.
Regarding mass transfer, it is preferable to establish a flow regime in the reactive phase, and thus in the liquid phase which contains dissolved gas, that is as close as possible to plug flow in order to obtain maximum conversion. However, in the case of highly exothermic reactions, plug flow generates a substantial temperature profile that renders thermal control difficult.
The mixture of reactants (hydrogen and carbon monoxide) entering the reactor undergoes the Fischer-Tropsch reaction and this continues as the fluid advances into the column. In this type of reactor function, the concentration and partial pressure of the reactants reduces along the reactor while that of the products (gaseous or liquid) and the water produced by the reaction increases. Plug flow is thus the origin of a concentration gradient associated, in the case of a highly exothermic or highly endothermic reaction, with a substantial temperature gradient along the reactor.
Consider the following reaction scheme, which is well known to the skilled person, for an exothermic reaction of the Fischer-Tropsch type: 
The undesirable parallel reaction, methane formation (reaction 2), has an activation energy (E2) that is higher than that (E1) of the principal hydrocarbon formation reaction. The rate of methane formation thus increases faster with temperature than that of the other hydrocarbons. Further, since the two reactions are exothermic (enthalpies xcex94H1 and xcex94H2 for reactions 1 and 2 are negative), progress of the reaction causes an increase in the heat released by the reaction, which increases the temperature and thus methanation.
An increase in the temperature gradient along the reactor thus results in a reduction in the selectivity for desired products.
The present invention describes a process whereby the formation of C2+ hydrocarbons, preferably C5+ and preferably mainly paraffins CnH2n+1 by reaction 1 is encouraged by controlling the parameters associated with the reaction.
In the case of the reaction scheme described above (reactions 1 and 2), the unsteady state material balance equations can be written as:                                                                         Species                ⁢                                  xe2x80x83                                ⁢                                  A                  :                                                                                    1                                                  Pe                          l                                                                    ·                                                                                                    ∂                            2                                                    ⁢                                                      C                            A                                                                                                    ∂                                                      Z                            2                                                                                                                -                                                                  ∂                                                  C                          A                                                                                            ∂                        Z                                                              -                                          (                                                                                                    r                            1                                                    ⁡                                                      (                                                          C                              A                                                        )                                                                          +                                                                              r                            2                                                    ⁡                                                      (                                                          C                              A                                                        )                                                                                              )                                                                                  =                                                ∂                                      C                    A                                                                    ∂                                      t                    *                                                                                                                                          Species                ⁢                                  xe2x80x83                                ⁢                                  B                  :                                                                                    1                                                  Pe                          l                                                                    ·                                                                                                    ∂                            2                                                    ⁢                                                      C                            B                                                                                                    ∂                                                      Z                            2                                                                                                                -                                                                  ∂                                                  C                          B                                                                                            ∂                        Z                                                              +                                                                  r                        1                                            ⁡                                              (                                                  C                          A                                                )                                                                                                        =                                                ∂                                      C                    B                                                                    ∂                                      t                    *                                                                                                                        Species          ⁢                      xe2x80x83                    ⁢                      C            :                                                            1                                      Pe                    l                                                  ·                                                                            ∂                      2                                        ⁢                                          C                      C                                                                            ∂                                          Z                      2                                                                                  -                                                ∂                                      C                    C                                                                    ∂                  Z                                            +                                                r                  2                                ⁡                                  (                                      C                    A                                    )                                                                    =                              ∂                          C              C                                            ∂                          t              *                                          
Similarly, the unsteady state energy balance can be written as:                     1                  Pe          T                    ·                                    ∂            2                    ⁢          T                          ∂                      Z            2                                -                  ∂        T                    ∂        Z              +                  (                                                            r                1                            ⁡                              (                                  C                  A                                )                                      ·                          (                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  H                  1                                            )                                +                                                    r                2                            ⁡                              (                                  C                  A                                )                                      ·                          (                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  H                  2                                            )                                      )            ·              τ                  ρ          ·                      C            p                                -                            U          ·          a          ·          τ                          ρ          ·                      C            p                              ·              (                  T          -                      T            cont                          )              =                                          ∂            T                                ∂                          t              *                                      ⁢                  
                ⁢        where        ⁢                  
                ⁢                  Pe          l                    -                                                  u              l                        ·            H                                D            ax                          ⁢                  xe2x80x83                ⁢                  Pe          T                    -                                                  ρ              ·                              C                p                                      λ                    ·                      u            l                    ·          H                ⁢                  xe2x80x83                ⁢        Z            -                        z          H                ⁢                  xe2x80x83                ⁢        τ              =                            H                      u            l                          ⁢                  xe2x80x83                ⁢                  t          *                    =                                                                  xe2x80x83                            ⁢              t              ⁢                              xe2x80x83                                      τ                    ⁢                      xe2x80x83                    ⁢                                    r              i                        ⁡                          (                              C                A                            )                                      =                              k            p                    ·                      ϵ                                          E                1                                            R                .                T                                              ·                      C            A            n                              
Given that the thermal dispersion very closely follows the mass dispersion, equality of the thermal Peclet number and the mass Peclet number for the liquid phase Pel constitutes a reasonable hypothesis that is accepted by the skilled person (P. L. MILLS et al., xe2x80x9cThree-Phase Sparged Reactorsxe2x80x9d in Topics in Chemical Engineering, volume 8, chapter 5, p. 364, K. D. P. NIGAM and A. SCHUMPE editors, GORDON and BREACH, publishers). The selectivity for product B and the temperature profile along the column can then be determined by solving the above equations. Solution leads to the results shown in FIGS. 1 and 2.